1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electronic copying machine, using a photosensitive drum, for instance, and, in particular, an apparatus for controlling the output of chargers located around the photosensitive drum.
2. Description of the Related Art
In an image forming apparatus using a photosensitive drum, an amount of charge on the photosensitive drum differs for each photosensitive drum used, even if the same voltage is applied to a charger. It is, therefore, necessary to control, in accordance with the respective photosensitive drum, the outputs of a charger for initial charging, a charger for transfer and a charger for separation, which are arranged around the photosensitive drum.
It has been the common practice, if the outputs of these chargers around the photosensitive drum are to be controlled, to control the supply voltage to the chargers in accordance with the surface potential measured by a surface electrometer on the surface of the photosensitive drum. It is, however, not advisable to provide the surface electrometer within the copying machine because it is large and expensive.
A means of controlling the outputs of chargers, using a special jig, has been developed to avoid the aforementioned drawback. In the case where the outputs of the chargers are to be controlled, the photosensitive drum is removed from the copying machine and an aluminum jig of a similar configuration is mounted there. A corona discharge is then created on the jig and a current value or voltage value obtained from the jig is measured for coincidence with the predetermined value. By so doing, a variable resistance across a high voltage transformer which is connected to the charger is manually controlled. However, the manual control operation using the aforementioned jig has proved difficult to use in comparison with an actual control on the photosensitive drum, and also time-consuming.
Since the size of the photosensitive drum is different from machine to machine, it has been necessary to prepare a plurality of jigs in accordance with the size of the photosensitive drum and change them accordingly. Therefore, a cumbersome control operation is required.